1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine coupled to a catalytic converter.
2. Description of the Related Art
Three-way catalytic converters (TWC) are commonly used to remove pollutants such as NOx, HC, and CO components in the exhaust gas of an internal combustion engine. NOx is removed from the exhaust gas by reduction using the CO, HC and H2 in the exhaust gas. There is also typically enough O2 present to oxidize the CO and HC. Generally, however, the catalyst used in such converters is able to remove the pollutants from the exhaust gas simultaneously only when the air/fuel ratio of the exhaust gas is kept in a narrow range near the stoichiometric air/fuel ratio.
FIG. 1 shows the conversion efficiency of a typical TWC as a function of measured exhaust gas air/fuel ratio. As can be seen in FIG. 1, TWCs require that the air/fuel ratio of the engine be held in a relatively narrow range, such as window 10, to assure high conversion efficiencies. Therefore, in order to reduce undesirable emissions within the exhaust gas, it is important to keep the air/fuel ratio of the engine in the region where the TWC has high efficiency. Typically, this is near the stoichiometric air/fuel ratio or at a predetermined offset near stoichiometric. Component drifting or aging can result in an altered air/fuel ratio and, hence, less than optimum efficiency of the TWC.
An object of the invention herein is to provide a method of locating the peak TWC efficiency window. Another object is to maintain engine air/fuel operation within the peak efficiency window of a catalytic converter.
An air/fuel control method for an engine including a NOx sensor in operative relationship to a catalytic converter. The method comprises the steps of providing a base fuel signal related to a quantity of air inducted into the engine and generating a bias signal for biasing the base fuel signal towards a leaner air/fuel ratio. The output of the NOx sensor is monitored to detect a predetermined exhaust gas NOx value representing a predefined NOx conversion efficiency. The base fuel signal is then modified as a function of the bias signal corresponding to the predetermined exhaust gas NOx value to maintain the catalytic converter within a desired efficiency range. In one aspect of the invention, the process of detecting the edge of the NOx conversion efficiency window is executed at predetermined time periods measured by the distance the vehicle has traveled, or the elapsed time since last base fuel value modification.
One advantage of the present invention is that it suppresses fluctuation in the air/fuel ratio. Another advantage is that it improves the efficiency of the catalytic converter.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.